"Do you have friends in class?"
Because it's not a specific friend we're asking about. は just sets the topic of discussion to friends, which is what the question is about. If you used が it would sound like you were referring to someone specific but who you couldn't refer to as anything better than "the friend", maybe playing some kind of guessing game about who the person is.
It seems like 授業（じゅぎょう） seems to mean "class" in the sense of "lesson". I'm not sure whether 授業 would be acceptable in this context.
I guess that's because the question is not about a friend physically in the classroom right now but sharing the same lessons every day
Particles often get dropped in informal speech, but technically some particle should be used in order to explain how the noun relates to the rest of the sentence. は (which is pronounced wa when and only when used this way) marks the topic, or scope of discussion.
It repeatedly marks me wrong for typing 友達 in full kanji instead of the weird half-kanji 友だち. It also marks が wrong and corrects it to は.
I was looking for a verb to put with imasu, because to me just imasuka means are there and not do you have.
Remember that います is really closer to "is present." We use it for many person-to-person relationships we use "have" for in English, like having a baby (赤ちゃんがいます) and having siblings (兄弟がいます)。